i had hip flexor problems a couple years ago
i did PT and massage in the morning but my butt was all tight just 15mins into my work day
turns out it was my desk chair
i changed it and voila all better

[aesop would suggest trying to figure out the source causing the tightness]

Yes I have had this happen a couple of months ago. I stretch my hip flexors a lot (both static and dynamic stretching), after runs, and sometimes after my run warmup. It does help. I also build my distances in my runs really slowly, which helps prevent injury. Are the distances on your training schedule new longer distances for you? If doing long distances are new for you, you may be increasing your mileage too quickly. If not, then it could be something else. There was a post awhile back and I think someone mentioned that something like that could also be a hydration issue.

How long have you been running (years/months)? Sorry to ask this as well, are you carrying extra pounds? Are you following a beginner marathon training program (if you are new to marathon distance)? These things can all make a difference with respect to susceptibility of injury. I have been running for about 8 years and I struggle following certain training programs for long distance running because I am susceptible to injury. I basically end up just making my own training schedule based on what my body can handle. For biking and swimming it hasn't been an issue at all. I've only been injured once from swimming, and it was only temporary, it healed within a couple of days and never returned. With running I have experienced more of the chronic type which is what you are describing. I haven't tried the foam rollers or massage, but either of these might be helpful. Hot baths with epsom salts also good for muscle pain relief. Also, there is probably a recovery routine that could also provide some relief too.

I would go slow in building your mileage and see how it goes.